docs: JAX installs by default — retire [jax]-extra install prose (PyAutoLens#702)

[Jammy2211]

Summary

Docs-only migration for JAX becoming a default dependency (PyAutoLens#702): install prose no longer recommends the retired [jax] extra (it survives upstream as a no-op alias, so nothing here would have broken — this is prose currency, not a fix). requirements.txt drops the extra suffix. Notebooks regenerated from scripts via generate.py; the markdown/ mirror pages received the identical sentence replacements by hand (running generate_markdown.py executes every curated script to re-capture figures — disproportionate for prose-only changes; the next curated regen produces this same text).

Hold for release (pending-release): the new prose states JAX installs by default, which is true only from the first release containing the upstream promotion.

Scripts Changed

• start_here.py (+ root notebook, markdown/start_here.md) — __JAX__ section: default install, no extra
• scripts/{imaging,point_source,group,multi_dataset,interferometer,multi_galaxy}/start_here.py (+ notebooks, markdown mirrors where they exist) — "if you installed autolens[jax]" → "JAX installs with autolens itself"
• scripts/guides/using_jax.py (+ notebook) — intro sentence updated
• requirements.txt — autolens[jax] → autolens

Note: markdown/imaging/start_here.md carries older surrounding prose than its script (pre-existing drift); only the [jax] clause was touched.

Upstream PR

PyAutoLabs/PyAutoLens#703

Test Plan

• Workspace smoke CI green on this PR (prose-only diff; CI runs every script)

Generated by the PyAutoLabs agent workflow.